Since the introduction of Integrated-Circuit (IC) chips, the conventional technique for mounting them on Printed-Circuit Boards (PCBs) has been the Pin-Through-Hole (PTH) technology. However, continual increases in IC-chip complexity, performance, and placement density are placing demands on the density and functionality of package interconnections influencing the development of various Surface-Mount-Technology (SMT) package-interconnection techniques to satisfy the needs, such as the Ball-Grid-Array (BGA) interconnection technique. These techniques are documented in an article by F. F. Cappo, et al. entitled "Highly Manufacturable Multi-layered Ceramic Surface-Mounted Package," IEEE/CHMT IEMT Symposium 1991, pp. 424-428, September 1991. Ball-Grid-Array is an area-array interconnection that can achieve a density of 400 interconnections per square inch. Because of the complexity or density of interconnections, a number of techniques have been developed to monitor interconnection quality.
Various automated solder-inspection systems are commercially available for monitoring solder-joint quality. The techniques used in these systems can be characterized by the radiation employed (either infrared, visible light, X-ray, or acoustic), the way in which the radiation interacts with the object being inspected, and the means used to detect the response of the radiation. The types of radiation can be subdivided into two broad categories, namely, non-penetrating and penetrating, depending on whether the radiation can penetrate the intervening chip-package material to image the Ball-Grid-Array joints. Techniques using penetrating radiation, such as acoustic and X-ray, can potentially inspect all Ball-Grid-Array joints within an area array including both peripheral joints and joints hidden under the chip-package material. The image generated by transmission systems (as opposed to cross-sectional systems) are due to the combined attenuation of the beam by every feature along its path. Therefore, the individual contribution to the attenuation of the beam by distinct features along the same beam path cannot be singularly isolated. For example, with respect to Ball-Grid-Array joints, the supporting solder ball with its high-lead content would entirely obscure the eutectic-solder fillets in a transmission image. For these reasons transmission systems tend not to be effective for the inspection of Ball-Grid-Array joints. However, cross-sectional inspection techniques have proven to be effective, as disclosed in the following patents:
U.S. Pat. No. 5,097,492 issued Mar. 17, 1992 and U.S. Pat. No. 4,926,452, issued May 15, 1990, both to Bruce D. Baker, et al., describe an inspection system using cross-sectional imaging to inspect microelectronic devices. U.S. Pat. No. 4,809,308 issued Feb. 28, 1989 to John Adams, et al. describes a transmission X-Ray inspection system.
Although cross-sectional imaging described above is useful for inspection, a number of interconnection or soldering defects are not reliably detected by these systems.
These references fail to disclose the use of the centroid of a cross-sectional image of an interconnection as a reference (location) for the measurement of a characteristic of the interconnection to determine the quality of the interconnection. The use of the image centroid of the interconnection has been determined to dramatically improve measurement accuracy in inspection so that defects and good joints can be more reliably distinguished, especially in the Ball-Grid-Array joint environment.
While Ball-Grid-Array (BGA) is compatible with existing assembly processes, and is functionally superior to Pin Through Hole (PTH), Ball-Grid-Array cannot successfully replace PTH unless it can also provide the same long-term reliability. The solder-joint volume and ball/pad alignment are the most critical characteristics that the assembly process for Ball-Grid-Array must consistently produce to ensure long-term reliability. Furthermore, the assembly process must be controlled to minimize such process defects as pad nonwets and solder bridges. To develop and control such a robust assembly process requires the use of an inspection technique to characterize the process by quantitatively measuring the critical characteristics of Ball-Grid-Array joints, such as the solder-joint volume and the ball/pad alignment. To this end, the overriding emphasis of inspection is to provide data on the assembly process that can be used to improve it in a closed-loop manner and not simply to screen the assembly-process output for defects. However, the Ball-Grid-Array technology poses a significant challenge to developing a satisfactory inspection process because the eutectic solder fillets that require inspection are obscured by the high-lead-content solder balls and a highly-metallized ceramic substrate. Thus, in order to reliably inspect the solder fillets, the inspection system must be able to isolate the solder fillets from the solder balls and the ceramic substrate. The method invention provides a suitable inspection system for these purposes.